Vox Cambridge 30 Reverb Twin keeps blowing TDA2050

[acorkos]

What am I missing? I found diode D3 shorted, so I replaced it, but the TDA2050 keeps blowing. All the voltages check out...

Thanks for your help!

[52 Bill]

Have you checked for any shorts in the output wiring?

[acorkos]

output wiring....do you mean at the phones/ext. spkr/line out PCB? The main output wire coming off pin 4 of the 2050 is not shorted to ground when tested out of the circuit.

if it helps any, the first blown 2050 (w/shorted D3) had legs 4/5 shorted. the second had 3/4 shorted

[Enzo]

I don;t generally expect to see either of those diodes fail in any amp. Replace both. Make sure 10 ohm R84 is not open. It is connected right there to pin 4 of the 2050.

There isn;t much else to this circuit.

Make sure the 2050 has good thermal contact witth the heat sink, but make sure also the insulator and plastic washer are in good shape.

Here is a pdf of the schematic.

[acorkos]

Make sure 10 ohm R84 is not open. It is connected right there to pin 4 of the 2050.

velly interesting....in this amp, R84 is a big 1 watt 10K resistor....looks like someone has been inside this amp before me and replaced this resistor with the wrong value.

will try a new 2050 tomorrow. thanks enzo

[Enzo]

Well, make sure the schematic covers your version. We want to make sure some revision didn't change the part numbers. ANd make sure I didn;t misread the drawing.

If that 10 ohm resistor isn;t there - and 10k is way to large for the job - the amp will be unstable and power oscillate until it dies.

Verify, does this resistor conect to pin 4 of the 2050? (the output pin)

[Robert M. Martinelli]

+1.

While you're there, you should also take the chance to replace the trem LDR ( LT 9914 ) and throw in an VTL5C3 ( nothing to do with your problem, just an improvement ).

The original LDR sucks, the new LDR will improve both reliability and the trem performance ( adjust the trem trimmers to your taste ).

Hope this helps

Best regards

Bob

[Enzo]

It was the LT9912 when they came out.

It has been some years ago now, but a major dealer I do service for got a bunch of the then-new Cambridge 30 amps, and every single one of them needed that opto to be replaced. A couple right out of the box, and some others a little while later, but every one nonetheless.

SO whenever I hear someone's Cambridge 30 has weak sound, distorted sound, no tremolo effect or extremely weak trem, I go right to that part - so far I have not been wrong.

[Robert M. Martinelli]

Yeah Enzo!
....And since he has the amp on the bench already, I think he should take the chance to throw a VTL5C3 in, even if the original LT99XX is still working, it will surely fail sooner or later, so why wait?

I own one of these amps myself, and I have to say that, once one has fixed their ( well known ) issues, they're pretty decent sounding amps.

Cheers

Bob

[acorkos]

the VTL5C3 has already been installed in this amp. getting the new 2050 today

[acorkos]

installed the new 2050.....working great now! thanks for the help everyone

[vipersassasin09]

the fuse in a cambridge 30 R i am working on blew... i hope this is all, but if not, what else could be the problem? the fuse was melted too i should say, the inside was anyway,... anyway, yeah...

[52 Bill]

the fuse in a cambridge 30 R i am working on blew... i hope this is all, but if not, what else could be the problem? the fuse was melted too i should say, the inside was anyway,... anyway, yeah...

Welcome to the board.

Fuses will sometimes die for no reason, but if it is really cooked, then I would suspect that there is a big short in the amp that has caused it to blow.

In the Cambridge 30, the output chip is always the first thing to check. Next would be power supply parts, like diodes caps, etc.

If your amp has a problem, you should start a new thread.

[vipersassasin09]

when adjusting the trem, what does each have an effect on? that is vr11 and vr12?

[Enzo]

Twiddle them until it sounds good.